Wavelength division multiplexing optical communications technology is used in the high-speed large-capacity optical networks that support the high level of data communications needed in today's society. There has been progression in the introduction of reconfigurable optical add drop multiplexers (ROADM devices) having reconfigurable add and drop functions at optical nodes, which are equivalent to branches in an optical network. To realize ROADM devices, wavelength selective switches (WSS), which can switch any wavelength in any direction, have gained attention. Light beam deflection elements which select a wavelength and deflect light beams to a desired output port have been used in wavelength selective switches. In Patent References 1, 2 and 3, something which utilizes the diffraction phenomenon by liquid crystal on silicon (LCOS) elements is proposed.
On the other hand, a great deal of research and development have been conducted regarding new modulation formats and increasing of transmission rates to respond to demand for transmission capacity in recent years, and optical networks have become more complex. To achieve the optimal filtering for the transmission rate and modulation format of each optical signal in such optical networks, dynamic control functions such as shifting of the median wavelength of the passband and enlargement and contraction of the passband are required in addition to conventional wavelength selection functions.
These functions can be realized by using high-precision LCOS elements or the like, in which a plurality of pixels are assigned to the channels of a WDM signal—that is, different wavelengths of light—and by using a multi-level optical phased array for light beam deflection.